Re: My Battery Monitor

[Lee Hart <leeahart@earthlink.net>]

Richard Bebbington wrote:
> If using relays to connect batteries to a central measuring
> device, please make sure your circuit CANNOT energise more than one
> relay at a time, or you'll short out part or all of your pack!
> ... fuses are a MUST anyway, but bear in mind that they all have to
> be rated to break full pack voltage, possibly at very high current.
> This is why I'd use relays - FETs are great, but they come
> with their own problems too.

I analyzed a Zivan Smoother, and they have exactly this problem. The
circuit is such that it can turn on more than one relay at once. And it
DID! Half a dozen relays have welded contacts, melted cases, blown
fuses, etc.

So for my Battery Balancer, I took the following approach. I used sealed
relays, on a PC board by themselves with only the coil diode, a fuse for
each battery wire, and connectors. Each relay board holds up to 8
relays. This way, the relay board can go in the battery box, minimizing
wiring.

The fuses are automotive type blade fuses. While they are only rated to
32v, the circuit is such that this is all they should ever have to
interrupt.

A triply-redundant system prevents more than one relay from pulling in
at once. First, the microcomputer's software commands a relay off, and
then checks the voltage with the A/D converter to see if it really
turned off.

Second, the micro's outputs go to CMOS 4028 decoder ICs, wired as a
1-of-N selector. The "0" state is "all off"; 1-8 are the relays on the
first relay board, 9-16 the next board, and so on. Thus it is impossible
for the micro to command more than one relay on.

Third, the relay coils are powered from an RC network. The R is higher
than the coil resistance; too high for a relay to pull in. The micro has
to command all relays off until the capacitor charges thru this
resistor, then it can command a relay on.

A fourth level of redundancy can be added if desired. Using DPDT relays,
the relay for battery N+1 can be wired through the NC contacts of the
relay for battery N. Like this (for 3 batteries):

                ____not used (all off)
           ____/____+ of battery 3
      ____/_________+ of battery 2
+____/______________+ of battery 1
     K1   K2   K3
                ____not used (all off)
           ____/____- of battery 3
      ____/_________- of battery 2
-____/______________- of battery 1

--
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen